The present invention relates to precision current references. More particularly, the invention relates to a network for generating a precision current reference that remains stable in high radiation environments.
Precision current references are used in a variety of electronic instruments, such as dynamically tuned gyroscopes (DTGs), interferometric fiber optic gyroscopes (IFOGs), force rebalance accelerometers, electromagnetically torqued gyros, and other inertial instruments requiring precision current excitation. Precision current references are also utilized in analog-to-digital (A/D) converters, digital-to-analog (D/A) converters, and precision laboratory test equipment.
Many devices, such as those listed above, must be capable of operating reliably in high radiation environments (i.e. greater than approximately 10.sup.13 n/cm.sup.2 and greater than 1 Mrad (Si) ionizing dose. Those devices typically require precision current references with stability ratings of better than one part per million (ppm) under high radiation conditions. Presently, there are no current references that meet this requirement. The present state-of-the-art radiation hardened references shift approximately ten ppm in high radiation environments.
Prior art current references include a variety of approaches. One approach is to use a precision voltage reference and then employ a transconductance stage to convert the voltage reference to a current reference. Typically, the voltage reference utilizes the reverse breakdown and/or the forward voltage characteristics of semiconductor diodes. It may also rely upon the bandgap voltage characteristics of a matched transistor pair.
Semiconductor diode and transistor implementations suffer from several drawbacks. One drawback is that the transconductance stage tends to degrade the performance of the current reference. Another drawback is that diode and transistor implementations require high quality single crystal semiconductor materials to achieve acceptable performance. However, the use of single crystal materials is problematic because exposure to neutron or proton radiation results in minority carrier lifetime changes caused by damage to the semiconductor crystal lattice. Another drawback is that the performance of diode based references are dependent upon small variations in the avalanche and tunneling current balance to achieve both temperature and radiation stability. Since the balance is dependent upon achieving a small difference between two large valued compensating mechanisms, avalanche and tunneling, these devices tend not to be sufficiently stable for precision applications in high radiation environments.
Some diode and transistor based devices attempt to circumvent this problem by pre-irradiating the devices at high neutron fluences. However, this tends to be expensive and is only partially effective.
An alternate prior art approach derives a precision current source from the negative resistance characteristics of degenerately doped semiconductor materials. A significant problem with this approach is that degenerately doped semiconductors suffer from electrical instability. This instability results from the negative resistance characteristics of the highly doped devices. Additionally, these devices are difficult and expensive to manufacture.
Another prior art approach relies on the volt-time product of square loop magnetic materials. Like the diode and transistor references, these devices require a transconductance stage to convert a voltage reference to a current reference. An additional drawback is that square loop magnetic materials have unacceptably high temperature coefficients. Some magnetic based devices, such as Superconducting Quantum Interference Devices (SQUIDs), avoid the temperature coefficient problem by operating at cryogenic temperatures. However, those devices are relatively large and heavy, rendering them impractical for many applications.
An alternate prior art approach utilizes self biased field effect transistors (FETs) to realize current regulator diodes. These devices are dependent upon both device geometry and on semiconductor doping levels. Moreover, they are sensitive to temperature variations and radiation exposure, both of which change leakage currents and effect the FET transconductance characteristic.
A general disadvantage to all of the above approaches, excluding the constant current diodes, is that they all require multiple precision support components. These components include precision operational amplifiers, precision resistors and high stability analog switches. The support components are typically employed to either excite the reference or to perform voltage to current conversions. The development of precision support components, to meet sub-ppm performance requirements, is difficult and costly, especially when operation is required in a high radiation environment.
By way of example, an operational amplifier, used to buffer a conventional precision voltage reference, must have an offset voltage stability of one microvolt to realize a one volt reference that is stable to within one ppm. Present state-of-the-art monolithic radiation hard operational amplifiers have offset voltage shifts in the range of ten to one hundred microvolts. Additionally, operational amplifier open loop gain changes of as little as six decibels cause ten to forty ppm errors for common inverting and non-inverting configurations.
Furthermore, the magnitude of the precision current in prior art devices is fixed. Real time control of the current magnitude requires additional complex, precision electronic components. Those components tend to further degrade the current reference performance.
Accordingly, it is an object of the present invention to provide a current reference that is stable when operating in a high radiation environment.
A further object of the invention is to provide an improved adjustable precision current reference.
Other objects, features, and advantages of the invention will be apparent from the following figures, description of the preferred embodiments thereof, and from the claims.